Modern transformer windings are fabricated using a wide variety of methods. In high power applications, one or more substantially rectangular shaped conductors are spirally wound about a core to form a coil. Often, the conductive cable itself is composed of a plurality of cable elements, with each cable element comprising a plurality of strands arranged side by side in a row.
The conductors used for transformer windings are typically fabricated in standard lengths. Therefore, for larger transformers, it is frequently necessary to serially join two or more conductor cables. Additionally, recent research has indicated that more efficient transformers can be produced by winding different portions of certain transformers with conductor strands having different strand configurations. For example, as described in U.S. Pat. No. 4,864,266, it may be desirable to wind the tap and end sections of a core form transformer with conductor strands formed differently than those used in the body section. In such circumstances, the two types of cables must be joined as well.
Two of the methods most commonly used to join conductor cables are brazing and crimping. While these two methods have proven to be reliable conductor joining solutions, they each have several drawbacks as well. In brazing, the end of one cable is overlapped with the beginning of the next cable and the joint is then brazed. Thus, the resultant joint is usually about twice the conductor thickness. Additionally, overlap brazing requires the thorough cleaning of all of the conductor insulation, which is somewhat difficult for multi-strand conductor cables. Insulation cleaning requires the use of a skilled operator, and the brazing operation itself introduces contaminants to the coil winding and heats both coil conductor and insulation.
Crimping typically uses a separate connecting piece that is crimped to the ends of the joined conductors. Like overlapping joints, crimped joints are also larger than the conductors themselves in either thickness or width. The connecting pieces required to form non-overlapping joints are undesirable because they require additional cost and are inconvenient. Since the size of the crimp connector will differ for different conductors, additional parts are required to accommodate different conductors. Crimp joints also require the removal of the conductor insulation prior to crimping.
One recently developed method of joining metals together uses an "ultrasonic" or "vibratory" cold welding process. To bond the metals together, high frequency mechanical energy is applied to the joint while pressure is applied to push the two parts being cold welded together. The vibratory energy causes one workpiece to undergo relative translating motion with respect to the other and induces a molecular interchange that can form a very strong bond when the vibrations are ended. Representative vibratory welding apparatus are described in U.S. Pat. Nos. 3,029,666; 3,791,569; 3,917,146; and 4,088,257. As described in U.S. Pat. No. 4,047,657, in many instances, insulation, an oxide film, paint, or other contaminants are present on the workpiece prior to the cold welding operation. It is therefore frequently necessary to clean the workpiece surfaces to be joined prior to inducing the molecular bonds. One cleaning approach is to divide the vibratory actions into two stages of operation. In the first stage, a rubbing action is provided to clean the surfaces of the workpieces to be joined. In the second stage, the molecular interchange is induced. Typically, the vibrational frequencies and amplitudes used for the two stages would be different. Such an approach works well when only thin layers of contaminants are found and the weld does not have to be continuous. However, when thick surface contaminations such as enamel coating are present on the workpiece surface, the contaminants tend to interfere with the quality of the welds.
Ultrasonic welding techniques have been used in numerous applications. For example, U.S. Pat. No. 4,401,252 discloses a method of connecting a starter motor armature coil to a commutator. U.S. Pat. No. 4,712,723 describes a method for bonding an insulated wire to a contact element. Specifically, during the ultrasonic welding operation, the wire's insulation layer is destroyed by mechanical action and the metallic parts are bonded by friction welding and concurrent deformation. However, the deformed area is susceptible to shearing. Accordingly, the '723 patent contemplates applying a suitable adhesive to the bond area to protect its mechanical connection. While such processes are apparently suitable for their intended tasks, they cannot be readily extended to the joining of transformer winding conductor cables which require a bond nearly as strong as the conductor cables themselves and preferably, the formation of a joint which is substantially the same cross section as the cables being joined.